1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly to a plasma display panel that is capable of preventing the discoloration of a substrate caused by migration of a metal bus electrode or metal paste's running down. Further, the present invention relates to a plasma display panel that is capable of improving a display quality.
2. Description of the Related Art
Generally, a plasma display panel (PDP) radiates a fluorescent body by an ultraviolet with a wavelength of 147 nm generated during a discharge of He+Xe or Ne+Xe gas to thereby display a picture including characters and graphics. Such a PDP is easy to be made into a thin-film and large-dimension type. Moreover, the PDP provides a highly improved picture quality owing to a recent technical development. Particularly, a three-electrode, alternating current (AC) surface-discharge type PDP has advantages of a low-voltage driving and a long life in that it can lower a voltage required for a discharge using wall charges accumulated on the surface thereof during the discharge and protect the electrodes from a sputtering caused by the discharge. Further, the PDP has advantages that its fabricating process is simple, it is easier to be made into a large screen and its response speed is fast because it does not have to form an active switching device every cell in the same way as a liquid crystal display (LCD).
Referring to FIG. 1, a discharge cell of the three-electrode, AC surface-discharge PDP includes a scanning electrode 30Y and a sustaining electrode 30Z formed on an upper substrate 10, and an address electrode 20X formed on a lower substrate 18.
The scanning electrode 30Y and the sustaining electrode 30Z include a transparent electrode 12Y or 12Z, and a metal bus electrode 13Y or 13Z having a smaller line width than the transparent electrode 12Y or 12Z and provided at one edge of the transparent electrode, respectively. The transparent electrodes 12Y and 12Z are formed on the upper substrate 10 and are made of indium-tin-oxide ITO. The metal bus electrodes 13Y and 13Z are formed by going through an etching process after depositing chrome/copper/chrome (Cr/Cu/Cr) by a deposition method, or by going through a patterning and firing process after printing photosensitive silver paste. On the upper substrate 10 provided with the scanning electrode 30Y and the sustaining electrode 30Z, an upper dielectric layer 14 and a protective film 16 are disposed. Wall charges generated upon plasma discharge are accumulated in the upper dielectric layer 14. The protective film 16 protects the upper dielectric layer 14 from a sputtering generated during the plasma discharge and improves the emission efficiency of secondary electrons. This protective film 16 is usually made of magnesium oxide (MgO). The address electrode 20X is formed in a direction crossing the scanning electrode 30Y and the sustaining electrode 30Z. A lower dielectric layer 22 and barrier ribs 24 are formed on the lower substrate 18 provided with the address electrode 20X. A fluorescent material layer 26 is coated on the surfaces of the lower dielectric layer 22 and the barrier ribs 24. The barrier ribs 24 are formed in parallel to the address electrode 20X to define the discharge cell physically and prevent an ultraviolet ray and a visible light generated by the discharge from being leaked into the adjacent discharge cells. The fluorescent material layer 26 is excited and radiated by an ultraviolet ray generated upon plasma discharge to produce a red, green or blue color visible light ray. An inactive mixture gas, such as He+Xe or Ne+Xe, for a gas discharge is injected into a discharge space defined between the upper and the lower substrates 10 and 18 and the barrier ribs 24.
Such a three-electrode AC surface-discharge PDP drives one frame, which is divided into a plurality of sub-fields having a different emission frequency, so as to realize gray levels of a picture. Each sub-field is again divided into a reset interval for uniformly causing a discharge, an address interval for selecting the discharge cell and a sustaining interval for realizing the gray levels depending on the discharge frequency. When it is intended to display a picture of 256 gray levels, a frame interval equal to 1/60 second (i.e. 16.67 msec) in each discharge cell is divided into 8 sub-fields SF1 to SF8. Each of the 8 sub-fields SF1 to SF8 is divided into a reset interval, an address interval and a sustaining interval. The reset interval and the address interval of each sub-field are equal every sub-field, whereas the sustaining interval and the discharge frequency are increased at a ratio of 2n (wherein n=0, 1, 2, 3, 4, 5, 6 and 7) at each sub-field. Since the sustaining interval becomes different at each sub-field as mentioned above, the gray levels of a picture can be realized.
By the way, the conventional PDP has a problem of discoloration of the substrate 10 caused by migration of the metal bus electrodes 13 and 13Z or the fact that silver (Ag) paste runs down the substrate 10 in case that the silver (Ag) paste is printed to form the metal bus electrodes 13Y and 13Z. The migration means that cation of silver Ag+ is eluted from an anode and moves to a cathode under dissolved oxygen in case of there being a voltage difference between two adjacent electrodes, which are the cathode and anode respectively. Sometimes, the cation of silver eluted discolors the surface of the substrate 10 in such migration process. The most significant cause of such substrate discoloration lies in an upper plate structure of the PDP. That will be described in detail in conjunction with FIGS. 2 and 3.
Referring to FIG. 2, metal bus electrodes 13Y and 13Z formed in a conventional PDP has their outer edge go in more by a certain length δ toward the center of a cell than the outer edge of transparent electrodes 12Y and 12Z located at the outer area of the cell. And the inner edge of the conventional metal bus electrodes 13Y and 13Z goes in more by a certain length t0 toward the outer of a cell than the inner edge of transparent electrodes 12Y and 12Z. There is a black layer 28 with conductivity formed between the metal bus electrodes 13Y and 13Z and the transparent electrodes 12Y and 12Z. The black layer 28 is formed by oxidizing metal or printing and patterning paste where metal powder and black pigment are mixed together. The black layer 28 act to prevent a contrast deterioration of a display screen caused by external light being reflected from the metal bus electrode 13Y and 13Z by absorbing the external light.
According to a structure of the metal bus electrodes 13Y and 13Z as in FIG. 2, the silver Ag paste is likely to run down to the transparent electrodes 12Y and 12Z or the substrates 10 so as to cause the substrate 10 to be discolored when the silver Ag paste is printed to form the metal bus electrodes 13Y and 13Z. This is because the outer edges of the metal bus electrodes 13Y and 13Z are close to the transparent electrodes 12Y and 12Z or the substrate 10. Further, anion of the metal bus electrodes 13Y and 13Z is likely eluted to discolor the substrate 10 by such a structure.
There is a PDP where an oxide film is formed on the substrate 10 as in FIG. 3 as another scheme for reducing the problem of the substrate discoloration.
Referring to FIG. 3, another conventional PDP includes an oxide film 30 formed of silicon oxide SiO between transparent electrodes 12Y and 12Z and a substrate 10. In this PDP, metal bus electrodes 13Y and 13Z has their outer edge go in more by a certain length δ toward the center of a cell than the outer edge of transparent electrodes 12Y and 12Z located at the outer area of the cell. And the inner edge of the metal bus electrodes 13Y and 13Z goes in more by a certain length to toward the outer of a cell than the inner edge of transparent electrodes 12Y and 12Z. There is a black layer 28 with conductivity formed between the metal bus electrodes 13Y and 13Z and the transparent electrodes 12Y and 12Z. The oxide film 30 is formed between the metal bus electrodes 13Y and 13Z and the substrate 10 so as to shut off for silver paste or silver ion eluted from the metal bus electrodes 13 and 13z not to move toward the substrate 10.
However, in case that the oxide film is formed on the PDP as in FIG. 3, because it has lower transparency than glass, the aperture ratio and brightness of the PDP is deteriorated and equipment and a process for depositing the oxide film should be additionally required.
Moreover, since the PDP as in FIG. 2 or 3 has the metal bus electrode 13Y and 13Z biased toward the inner side of a discharge cell, so there is a problem of the aperture ratio being that much smaller.
Particularly, since the light is leaked into the direction of adjacent discharge cells C1 and C2 through a boundary area BD between the adjacent discharge cells C1 and C2, the PDP in FIGS. 2 and 3 has a bad contrast, and since the light incident by the black layer 28 via metal bus electrodes 13Y and 13Z is shut off, a brightness is deteriorated, thereby having a problem of a bad display quality as shown in FIG. 4.